The long-term goal of the work undertaken in the P.l.'s laboratory is to understand how retinal ganglion cells participate in vision. The goal of the work proposed in this application is to model how two types of ganglion cell - X and Y cells (Enroth-Cugell & Robson, 1966) - participate in the representation of visual images. X and Y cells form the vast majority of cat ganglion cells with center-surround receptive fields and are a major component of the retinogeniculate pathway. This is the first part of the principal visual pathway in higher mammals including humans. At the level of the ganglion cells, there are fewer neurons to encode visual images than at any other level in the visual system. This economy of neuron number brings a penalty. Damage to retinal ganglion cells produces a large detrimental effect on visual function. In fact, many diseases of the retina (e.g., glaucoma) attack visual performance by the destruction of ganglion cells. Hence, an understanding of the role of retinal ganglion cells in vision is in general critical. Secondly, one of the most valuable diagnostic tools in the field of neuro-ophthalmology is the visual field test. There can be little question that the results of this project would find direct application in the improved design of perimeters and the interpretation of visual fields, leading to an improvement in patient care.